1. Field of the Invention
The present invention relates to an image-signal processing device for performing processing in which image signal noise generated by pixel defects and the like in an image sensor is reduced.
2. Description of the Prior Art
Digital cameras and other imaging apparatuses generate image signals using CCD (charge coupled device) image sensors or other solid-state image sensors. Pixel size has been reduced due to the increasingly higher pixel counts of solid-state image sensors, and difficult manufacturing processes are necessary. Due to these circumstances, pixel defects are not uncommon in the solid-state image sensors used in imaging apparatuses. Pixel defects include black-spot pixels that produce signal charges at a low rate relative to the incident light, and white-spot pixels that produce excessive signal charges due to dark current and the like. Screen noise is generated from the appearance of these black and white spots on the screen. This pixel-defect noise can be rendered unnoticeable by performing correction using image signal processing, and image quality can thereby be ensured.
The dark current noise that is generated by lattice defects and the like in the semiconductor substrate that forms the solid-state image sensor is also significant source of screen noise. Randomly produced dark-current shot noise causes roughness in the image quality. This dark current noise can also be removed by image-signal processing devices, and a variety of processing methods have been proposed for appropriately removing this noise.
FIG. 1 is a block diagram that shows the configuration of a conventional image-signal processing device, wherein image data is generated from performing pixel-defect corrections, noise suppression, and contour correction on the basis of an image signal output from an image sensor 2. The image sensor 2 is, e.g., a CCD image sensor, and the image signal output from the image sensor 2 is converted into digital image data by an A/D converting circuit 6 after being subjected to sample-and-hold and other processes in an analog-signal processing circuit 4. A digital-signal processing circuit 8 retrieves the image data from the A/D converting circuit 6 and performs various processes.
The digital-signal processing circuit 8 comprises a defect-correcting part 10, a noise-suppressing part 12, and a YC separation part 14. The defect-correcting part 10 performs processes for correcting the aforedescribed pixel defects. For example, the defect-correcting part 10 designates pixels having extremely low pixel values relative to surrounding pixels as black spots; designates pixels having extremely large pixel values relative to surrounding pixels as white spots; and substitutes corrected values for the pixel values of the black-spot and white-spot pixels. The corrected values are determined on the basis of the pixels values of the pixels surrounding the respective black-spot and white-spot pixels.
The noise-suppressing part 12 performs processes for suppressing roughness on the screen due to the aforedescribed dark current noise and the like. The noise-suppressing part 12 constitutes, e.g., a median filter, a mean filter, or another type of two-dimensional filter and reduces or removes noise components that differ with each pixel. The pixel range that is prone to the effects of dark current has the property of expanding according to increases in exposure time and temperature. The image-signal processing device proposed in Japanese Laid-open Patent Application No. 2006-13948 is an example of a noise-suppressing circuit for handling the possibility of expansions of the regions that produce such noise.
The YC separation part 14 receives the input of image data that has been subjected to de-noising processes in the defect-correcting part 10 and the noise-suppressing part 12; performs gamma correction and contour correction on the basis of this image data; and generates a brightness signal Y. The YC separation part 14 also performs chroma processing on the basis of the image data input after de-noising and generates color-difference signals U, V.
The contour correction, the generation of the brightness signal Y, and the color-signal generation of the color-difference signals U, V performed in the conventional YC separation part 14 are carried out using image data that has been subjected to de-noising processes in the defect-correcting part 10 and the noise-suppressing part 12. In other words, in conventional configurations, all of the aforementioned processes of the YC separation part 14 are performed on the basis of image signals (image data) of the same de-noising level. The de-noising effect increases if the de-noising level is set high, but on the other hand, spatial changes in the pixel values other than noise are also more likely to be smoothed out and compromised. Problems have therefore arisen in that, conventionally, when the de-noising level is raised, the resolution of the image deteriorates, contour correction is not appropriately performed, and the edges are disturbed.